At no point did it cross your mind that a potential alliance with the Eldar would only be if the situation required it?. He is hardly going to allow an Eldar to kill his battle-brothers nor is he going to ally with any Xeno unless the situation is most dire. We can handle a few hundred Orks, Eldar and Tyranids but Chaos Space Marines may become an issue if they are imbued with corrupted weaponry and/or rewards for worship to a specific chaos god and large numbers of the previously listed species may also stand against us. At that point, i will albeit somewhat unhappily request assistance for the Eldar, and this would only last until the goal of the Emperor's finest is fulfilled, at that point we offer them their fair chance to retreat for their assistance. Or we purge them from existence if they fail to heed our command.
I Trust In My Brother as i trust in the emperor.. i know he wont betray me.. nor will he betray his chapter..
All the more reason the iron hands can't trust any one but their own chapter brothers. This coming war will be fought from all sides and maybe even from with in our own faction brothers.
This is a marine I would fight both with and against with honor due to a true warrior on the field of battle. One who knows that an alliance is only working together for a time to benefit both sides. I would honor this warrior by fighting at his side and offering him the same at the end of the battle should we be on Eldar lands a chance to leave unharmed. There is no shame in working together at a common goal, it is more a sin to be so prideful as to let your enemies beat you because you are unwilling to set aside your pride for a time and work together with someone who is a former foe. After all the enemy of my enemy I can call an ally the question is can you?
No me and my chapter brothers have already discussed and decided that if we are forced to fight next to xenos ,we will kill them as soon as those brothers gave you safe passage we will kill the xenos when they think their safe. We the iron hands follow the emperor's rules that were set so long ago during the crusade. My battle brothers may have forgotten the rules of engagement but we the Iron Hands do not.